FL7740
www.onsemi.com
9
APPLICATION INFORMATION
General
FL7740 is high power factor flyback controller with
accurate primary side constant voltage regulation for
smart LED lighting and AC-DC adapter, TV & monitors
application. Precise output voltage detection and
dynamic function manage good CV regulation. Startup is
fast with internal HV biasing circuit with overshoot-less
gain control. It guarantees high system reliable
protection functions such as output over voltage, output
short, over load, over current and thermal shut down
protections.
Constant Voltage Regulation
VS pin detects output voltage information (=V
EAV
)
during secondary side diode conduction time and
internal gm amplifier regulates the detected voltage at
3.5 V.
Dynamic Response at Load Transient
At load transient condition, V
EAV
is shortly out of
regulation due to the narrow PFC loop bandwidth. When
V
EAV
is far from 3.5 V regulation reference, duty is
quickly changed to bring the V
EAV
back to 3.5 V by
dynamic control function.
HV biasing at startup
Internal HV biasing circuit quickly charges external
VDD capacitor to begin IC operation at plug-in. After
500 ms initial time, HV biasing stops for low standby
power.
Overshoot-less gain control at startup
Once IC operation starts, feedback loop is dominantly
controlled in proportional gain to speed up the output
capacitor charging. Once output voltage is settled down
close to the regulation target, gain control is smoothly
changed to integration gain with no output voltage
overshoot.
Digital PF optimizer
FL7740 compensates input current phase shift caused by
EMI filter capacitor current in a half line period. With
sophisticated digital PF optimizer, FL7740 significantly
improves power factor in the wide load range.
Pulse-by-pulse current limit
When CS pin voltage reaches to 1.2 V current limit
reference, GATE turn-on is terminated to limit primary
peak current.
Auto Restart at Protection
Once protection is triggered, IC operation stops for 3 sec
and begin the operation for auto restart.
Output Short Protection
When V
EAV
is less than 0.7 V continuously for 35 ms,
output short protection is triggered.
Output Over Voltage Protection
When V
EAV
is higher than V
VS-OVP
threshold or VDD is
higher than V
DD-OVP
, output over voltage protection is
triggered.
Output Diode Short Protection
Once output diode is short circuited, high di/dt in the
primary winding is occurred by leakage inductance.
Once CS pin voltage reaches to 1.7 V, switching is shut
down.
Sensing Resistor Short Protection
At first switching, sensing resistor short condition is
monitored by detecting CS pin voltage. If CS is less than
75 mV during first GATE turn-on time, sensing resistor
short protection is triggered.
Over Load Protection
When output is over loaded, pulse-by-pulse current limit
event is occurred. If this event lasts for 60 half line
cycles, over load protection is triggered.
Thermal Shut Down
If internal junction temperature is higher than 150ºC,
protection is triggered and released with 30ºC hysteresis.
